#!/bin/bash
# Scriptname: do_increment
 increment ()  {
    local sum;     # sum is known only in this function
    let "sum=$1 + 1"
    return $sum    # Return the value of sum to the script
} 
echo  -n "The sum is "
increment 5    # Call function increment; pass 5 as a
               # parameter; 5 becomes $1 for the increment
               # function

echo $?        # The return value is stored in $?
echo  $sum     # The variable "sum" is not known here
